Auto-levelling geophone development and testing
Malcolm B. Bertram, Eric V. Gallant, Robert R. Stewart
An auto-levelling, motion sensor (multi-component geophone) is developed and tested. The geophone elements are mounted in a sphere free to rotate inside a spherical cavity in the sensor case. Both one-component (vertical element) and three-component (one vertical and two horizontal elements) devices have been built and tested. A testing apparatus has also been developed. It consists of a large speaker coupled to a platform upon which the geophone device is attached. Reference elements as well as a laser interferometer provide calibration signals for the device under test. Conventional geophones have sinusoidally decreasing output when they are tilted from the vertical (about 70% of maximum when tilted 40 degrees from vertical). The auto-levelling device has a vertical geophone output that decreases less than 3% under similar tilt for frequencies from 10Hz to 100Hz. Horizontal motion, as measured by the auto-levelled elements, is within 3% of the calibrating horizontal element for frequencies from 10Hz to 120Hz. Penduluming of the inner sphere does not appear to be a problem above 2Hz. The auto-levelling sensor shows considerable promise to improve conventional seismic recording and fully capture multicomponent seismic motion.